Exhaust turbochargers for pressurizing charging air of engines by utilizing exhaust gas energy of the engines, especially those used in engines for automobiles that are equipped with waste gate valves, which allow a part of the exhaust gas from the engine to bypass the turbine of the exhaust turbocharger from the entrance passage to the outlet passage of the turbine housing via a bypass passage by opening the waste gate valve for opening/closing the bypass passage so that exhaust gas energy imparted to the turbine rotor is reduced in order to prevent excessive increase of charging air pressure exist.
FIGS.4 to 7 show an example of a twin scroll type exhaust turbocharger having a waste gate valve disclosed for example in Japanese Laid-Open Patent Application No.11-44219 (patent literature 1), FIG.4 being a longitudinal cross sectional view of the substantial part of turbine side of the turbocharger, and FIG.7 being a partial cross sectional view of the waste gate valve attached to the turbine housing.
Referring to FIGS. 4 to 7, the turbocharger comprises a twin scroll type turbine housing 1 having two exhaust gas entrance channels, a bearing housing 1a fixed to the turbine housing 1, and a turbine rotor 2. A turbine shaft 4 of the turbine rotor 2 is supported for rotation by the bearing housing 1a via a bearing 5.
The turbine housing 1 has two exhaust gas entrance channels 03a and 03b connecting to two scroll passages 30a and 30b for guiding the exhaust gas to the entrance of the turbine rotor 2, exhaust gas bypass passages 3a and 3b, and an exhaust gas outlet passage 7. Reference numeral 8 is a space for opening the bypass passages to and it is a part of the exhaust gas outlet passage 7, so hereafter it is referred to also as the exhaust outlet passage 8. The exhaust gas bypass passages 3a and 3b are partitioned by a partition wall 3c of the turbine housing 1 and open into the exhaust gas outlet passage 8.
In FIG. 7, a waste gate valve 10 is provided at the ends of the exhaust gas bypass passages 3a and 3b opening into the exhaust gas outlet passage 8 to connect the exhaust gas entrance channels 03a and 03b to the exhaust gas outlet passage 8 bypassing the scroll passages 30a and 30b in order to open/close the bypass passages 3a, 3b. 
The waste gate valve 10 includes a plate-like valve body 11 for opening/closing a pair of the exhaust gas bypass passages 3a and 3b, a lever 12 connected to the valve body 11, and other component parts. The waste gate valve 10 is composed such that the exhaust gas bypass passages 3a and 3b are closed or opened by allowing the valve body 11 to be seated against the seat face 3d of the turbine housing 1 at the opening end of the pair of the exhaust gas bypass passages 3a and 3b or to be departed from the seat face 3d by means of the lever 12 by actuating a waste gate valve drive device not shown in the drawings.
The waste gate valve experiences vibrations of the engine and exhaust gas pulsation.
In the twin scroll type exhaust turbocharger having a waste gate valve disclosed in patent literature 1, as shown in FIGS. 6 and 7, two exhaust gas bypass passages 3a and 3b are partitioned by a partition wall 3c, and the flat seat face 11s of the valve body 11 of the waste gate valve 10 is brought into close contact with the annular seat face 3d surrounding the pair of the exhaust gas bypass passages 3a, 3b and the end face 3f of the partition wall 3c, the end face 3f being finished level with the seat face 3d, when the waste gate valve 10 closes the exhaust gas bypass passages 3a, 3b. 
When machining the seat face 3d to be level with the end face 3f, the end face 3f of the partition wall 3c of relatively small thickness partitioning the bypass passages 3a, 3b is machined together with the annular seat face 3d surrounding the pair of the bypass passages, so the end face 3f, particularly central part thereof of the partition wall 3c tends to be protruded from the seat face 3d. If the end face 3f is protruded from the seat face 3d, the flat seat face 11c of the valve body 11 contact the end face 3f of the partition wall 3c before it comes to contact with the annular seat face 3d when the valve body 11 closes the bypass passages and a clearance is formed between the seat face 11c of the valve body 11 and the annular seat face 3d of the turbine housing 1. As a result, the valve body 11 swings with the extending line of end face 3f of the partition wall as a fulcrum axis when the valve body 11 closes the bypass passages. That is, the valve body 11 can not completely close the bypass passages, and vibration of the valve body 11 occurs accompanying chattering noise. Further, exhaust gas leak through the clearance between the seat face 11c of the valve body 11 and the annular seat face 3d of the turbine housing occurs, resulting in decreased utilization of exhaust energy of the engine.